One of the thermionic detectors employed in gas chromatography, primarily for nitrogen and phosphorous detection, employs a heated source comprising a glass bead containing an alkali metal such as rubidium or cesium. The bead is formed on a platinum wire and is positioned at the outlet of a chromatographic column. It is heated to provide a source of alkali metal ions. One method of heating is by means of an electrical current which is passed through the platinum support wire. When employed as a nitrogen detector in gas chromatography, the voltage across the bead must be stable to one part in twenty-five thousand (1:25,000) or better, with a system time constant of 50 msec, in order to obtain the desired detection limit.
One method used in the prior art for heating the detector bead employs closed loop d.c. control. A desired current of approximately 5 amperes through the bead, combined with a voltage across the bead of approximately 0.7 volt, results in a bead power requirement of 3-3.5 watts. However, in order to achieve that, it was necessary to start out with a 5 volt power supply. This is very inefficient in that the power supply transformer requires approximately 50 watts on the primary side in order to achieve the needed 3.5 watts at the bead. Not only is the circuit very inefficient but bulky components are required in order to dissipate the energy.
Another prior art approach employs a transformer which is a.c. coupled to the bead and wire in an open loop configuration. The voltage drop across the wire and bead is not measured. Instead, the electrical pulses supplied to the transformer are measured, but without feedback. Accordingly, it is very sensitive to any inductance in the circuitry and this makes the mechanical design difficult. Furthermore, the a.c. coupling is inefficient and the design allows for no compensation for ambient temperature changes in the transformer or load.
Accordingly, it is a primary object of the present invention to provide an electrical heating circuit for a thermionic detector source which has improved efficiency and fewer mechanical design constraints. Other objects, features, and advantages will become apparent from the following description and appended claims.